Multiple methods of inducing permanent allograft tolerance have been devised in animal models using monoclonal antibodies directed against a variety of cell surface molecules. However, the cellular and molecular mechanisms that govern tolerance induction in these systems are not understood. A possible mechanism of tolerance induced by monoclonal antibodies is that uncommitted T helper cells are stimulated by receptor ligation to preferentially differentiate into a TH2 phenotype that promotes graft tolerance. The TH1/TH2 model of T cell differentiation holds that uncommitted TH0 cells are stimulated to differentiate into one of two distinct phenotypes: TH1 cells that respond to IL-12 and secrete primarily IL-2 and IFN gamma, or TH2 cells that secrete primarily IL-4, IL-10, and IL-13. Experimental evidence in models of autoimmunity and allograft rejection associate TH1 cells with progression of autoimmune disease and allograft rejection, and TH2 cells with the suppression of autoimmunity and induction of allograft tolerance. However, it is not yet known how the immune system normally determines whether cells will develop into one subset or the other. Furthermore, the assertion that TH2 cells suppress graft rejection is disputed by some investigators, and it is not clear what determines the protective versus adverse effects of any one T cell subset. Preliminary evidence from our laboratory indicates that tolerance induced by sequential ligation of the CD2 and CD3 receptors is dependent on both IL-4 and IL-10, but not IL-2 and does not develop in IL-4 knock-out animals. Adoptive transfer of naive lymphocytes to the IL-4 knockout animals restores the ability to develop tolerance in the IL-4 knockouts. Others have shown that tolerance induced by CTLA4Ig blockade of CD28-B7 and by combined treatment with anti-CD4 and anti-CD8 monoclonal antibodies is also associated with the induction of TH2 cells and cytokines. Our hypothesis is that antigen specific tolerance resulting from sequential ligation of CD2 and CD3 receptors induces TH0 cells to differentiate into TH2 cells while concomitantly inhibiting TH1 development. The first aim of this investigation will define whether TH2 cytokines are critical for the induction of tolerance by anti-CD2 plus anti-CD3 treatment by examining whether this treatment can induce tolerance in strains of knockout mice that are deficient in the ability to generate either TH1 or TH2 responses. The second aim will examine the immunologic responses at the cellular and molecular level in the knockout animals and compare these responses in the setting of effective and ineffective tolerance induction to provide insight into the mechanism of tolerance induction. The third aims will define when TH2 cytokines are necessary for tolerance induction. The fourth aim will determine which cell type or types are responsible for secreting the necessary cytokines.